Microelectromechanical (MEMS) gyroscopes are used to detect angular rotation about an input axis by measuring Coriolis forces exerted on a number of resonating proof masses. A typical MEMS gyroscope includes two silicon proof masses mechanically coupled to a silicon or glass substrate by suspension springs. A number of recesses etched into the substrate allow selective portions of the silicon structure to move back and forth freely within an interior portion of the device. A pattern of metal traces formed on the substrates can be used to deliver various electrical bias voltages and signal outputs to the device.
The drive system for MEMS gyroscopes typically includes a number of drive elements that cause the proof masses to oscillate back and forth along a drive axis perpendicular to the direction in which Coriolis forces are sensed. A motor mode of the gyroscope may comprise two proof masses moving at equal but opposite velocities in a direction substantially parallel to the substrate and along a line connecting the centers of the proof masses. In some applications, the motor mode of the proof masses can be driven electrostatically at its resonant frequency using a number of interdigitated comb drive fingers adapted to convert electrical energy into mechanical energy by electrostatic actuation. When the gyroscope is rotated about its input axis perpendicular to the drive axis, the motor mode velocity of the proof masses produces a Coriolis force that drives the proof masses along a sense axis perpendicular to the drive axis and input axis.
The sensing system of the gyroscope may include one or more sense electrodes that can be charged with a DC sense bias voltage to produce an electric field in the spaces between the sense electrodes and proof masses. A sense resonant mode of the gyroscope typically includes movement of the two proof masses at equal but opposite velocities along the sense axis. The Coriolis force due to the motor velocity drives the sense resonant mode, typically at or near the frequency of motor motion. In some designs, the Coriolis force drives the sense mode off-resonance.
As each proof mass moves back and forth above the substrate, the Coriolis force resulting from rotation of the gyroscope about the input axis causes the spacing between the proof masses and sense electrodes to vary. The displacement of sense resonant mode motion can then be determined capacitively by detecting the current induced on the proof masses due to the time-varying sense capacitance. By measuring the output current produced on the proof masses, a measure of the rotational motion and/or acceleration of the gyroscope can be ascertained.
A significant source of errors in many MEMS-type gyroscopes is attributable to quadrature motion of the proof masses, defined as motion along the sense axis 90° out-of-phase with the motion produced by the Coriolis force. The resultant mechanical feedthrough signal caused by such quadrature motion is often referred to as the quadrature signal, and typically includes an AC output signal of the gyroscope that is 90° out-of-phase with the signal produced by the Coriolis force. Such quadrature may result, for example, from imperfections in the profile of the comb fingers and suspension springs used in the drive system, and from other imperfections created during the manufacturing process. Such imperfections or errors can result in the motor motion producing a quadrature force on the sense mode motion that is in-phase with the motor displacement, and therefore out-of-phase with the motor velocity.
The quadrature force may be several orders of magnitude greater than the smallest detectable Coriolis force, affecting the ability of the gyroscope to accurately discern subtle variations in the rate signal. As a result, additional error correction circuitry is typically required to remove the quadrature signal from the output sense signal. While the effect of the quadrature force on the gyroscopic rate output signal is typically reduced by the fact that it is 90° out-of-phase with the Coriolis force, small phase errors in the inertial sensor and associated electronics can nevertheless produce errors in the rate output signal, diminishing the ability of the gyroscope to accurately detect and measure rotation.